Magnetic shaping process



y 1963 D. H. BIRDSALL ETAL 3,088,200

MAGNETIC SHAPING PROCESS Filed Nov. 10, 1960 INVENTORS. DALE H. BIRDS/ILL ROBERT E. RILEY ATTORNEY ater dfififilfiii Patented May 7., 1963 fiice 3,088,200 MAGNETIC SHAPING PROCESS Dale H. Birdsall, Livermore, and Robert E. Riley, Hayward, Calif, assignors to the United States of America as represented by the United States Atomic Energy Commission Filed Nov. 10, 1960, Ser. No. 68,580 4 Claims. (Cl. 29-421) The present invention relates to the field of metal working in general and in particular to a process for shaping electrically conductive materials by the utilization of forces exerted upon the material by an intense pulsed magnetic field.

With the possible exception of casting and machining all metal shaping processes operate on the principle of applying pressure to the metal surface in order to obtain plastic deformation. In conventional shaping processes such as forging, rolling and swaging the applied pressure is exerted unidirectionally, i.e., parallel to the stroke of the press or roll. Since the pressure is not distributed uniformally over the surface of the material being worked, internal strains are frequently set up Within the grain structure of such material. The removal of these grain defects or strains often requires extensive heat treatment which is a costly and time consuming process.

Further shortcomings prevail in prior art methods of metal forming such as, for example, the difficulty experienced in conventional methods of forming thin-walled objects or objects with intricate shapes. Such conventional methods are known generally as drawing, squeezing or spinning processes of metal forming. When performing drawing or squeezing operations two dies must normally be employed to form the material therebetween into the desired shape. In the process known as spinning, generally used to form thin Walled objects, only one inside die is necessary but the material to be formed must be forced to conform to the shape of the die or mandrel by utilizing a lever, thereby involving a certain amount of manual effort and skilled labor for proper performance thereof. Furthermore, the conventional metal shaping methods of previous mention do not readily lend themselves to the forming of intricately shaped objects.

However, the method set forth in accordance with the present invention overcomes the above-mentioned short-comings. That is, the forming of not only usual but unusual and intricate shapes or thin-walled objects may be accomplished with a minimum of internal strains being created within the material being formed. The foregoing advantages are possible in the present invention since the pressure exerted against the material to be formed is not the result of a solid press or roll but is instead created by a compressible magnetic field. That is, a magnetic field, much in the nature of a gas, will conform to the surface of the object to be formed, thereby creating pressure uniformly and simultaneously over the entire surface of the material to be formed resulting in a minimum of internal strains being created therein.

Therefore it is an object of the present invention to provide a relatively simple process for shaping electrically conductive materials.

It is another object of the present invention to provide a process wherein the energy of a magnetic field is utilized to shape electrically conductive materials.

A further object of the present invention is to provide a process wherein a magnetic field is employed to simultaneously exert substantially uniform pressure against and generate heat within a material being shaped.

It is still a further object of the present invention to provide a process whereby intricate objects may be formed of metal with a minimum of internal stresses produced therein.

Other objects and advantages will be apparent in the following description and claims considered together with the accompanying drawing in which the single FIGURE 1 is a perspective view of apparatus for conducting the method of the present invention.

Briefly, the present invention provides a method of shaping material wherein the material to be shaped, generally in the form of a metal tube or sheet of suitable proportions, is first disposed around a suitably shaped mandrel of substantially lower electrical conductivity than the conductivity of the material to be shaped. The combination mandrel and material assembly is then placed in a high flux density pulsed magnetic field. The pres sure of the magnetic field deforms the material to the shape of the materially lower conductivity mandrel therewithin. As is well known in the art such pressure arises due to the fact that a suddenly rising magnetic field does not immediately enter into the conductive tube or sheet being formed but instead exerts a force thereagainst.

It should be noted that at such time as the magnetic field is created, the generation of eddy currents within the material gives rise to ohmic heating therein. As is well known in the art of metal working, material shaped under hot-worked or annealing temperatures remains rather ductile and has a minimum of internal strains created therein by the working operation. In the present method of metal forming, an object formed with a single pulse of the magnetic field would be so formed before it felt the effects of the heat produced therein by the magnetic field. Therefore, the annealing effect created by magnetic field heating, is not in evidence during the application of the initial forming pulse. However, the annealing effect is present where the present method is operated as a series of successive shaping operations. In such a series of operations the temperature of the material being shaped is readily made to approach the annealing temperature thereof and the advantages presented by such an effect are realized.

The method of the present invention is described in greater detail with reference to the figure wherein for purposes of illustration and description only, a simple cylindrical mandrel 11 having a central necked-down portion 12 on its surface is utilized in the forming process. It is noted that mandrel 11 may be of any other shape depending upon the form of the desired completed object. Such mandrel 11 is formed of a non-conducting material and is coaxially disposed within a solenoid 13. The material to be formed preferably is provided as a tubular conductor 14 and is coaxially disposed about the mandrel 11 and between such mandrel and the solenoid 13. The entire apparatus is enclosed by a cylindrical housing 16, which housing is provided to add structural strength to the solenoid coil and thus prevent it from expanding due to the intense pressure created radially outward thereupon by the pulsed magnetic field resulting from energization of the coil. A capacitor bank 17 and high current switch 18 are connected in series with solenoid 13 to provide the circuitry for introducing the necessary electrical energy, at the proper instant, to the solenoid for operation thereof.

With reference to the conduct of the method in the formation of a thin-walled object having a shape conformed to that of the mandrel 11 the material to be formed, provided as the tubular conductor 14, is coaxially disposed between the mandrel 11 and the solenoid 13 as per previous description, and as indicated in full line in the figure. The switch 18 is closed as is shown in phantom line in the drawing and the previously charged capacitor bank 17 is discharged almost instantaneously into solenoid 13. The rapidly rising magnetic field of the solenoid induces eddy currents in conductor 14. The direction of these induced currents is such to create a second magnetic field which opposes the increase in the solenoid field. As a result the magnetic flux density in the annular space between solenoid 13 and conductor 14 reaches an extremely high value in a short time interval. Since the high flux density does not immediately penetrate the conductor 14 it thus gives rise to a uniform force directed substantially radially inward over the entire surface of such material. As a result of the radial, inwardly directed force the material is deformed into the necked-down portion 12 of the mandrel 11 to thus assume with one large magnetic pulse the overall surface configuration as indicated in phantom line in the figure.

However, in the forming of more intricate objects it may be advantageous to apply the shaping process of the present invention as the series of operations previously mentioned, wherein a number of relatively small magnitude, magnetic field pulses are applied in continuing succession to the object. In this way the annealing temperature of the conductor 14 is approached and rendered ductile while simultaneously being incrementally urged towards the mandrel to ultimately conform to same. The ductility allows the forming of intricate objects without the need to interrupt shaping operations in order to conduct a separate, time-consuming and relatively expensive annealing process thereon.

After the material of conductor 14 is formed about the mandrel 11 the combination material and mandrel assembly is removed from within the solenoid. The mandrel is thereupon removed from within the formed material by suitable means, e.g., dissolving the mandrel with a solvent in which the material is insoluble. The mandrel 11 may be constructed of several close fitting pieces, which construction comprises an alternative method for facilitating removal of the mandrel from inside the formed material.

As an example of the forming method of the invention, a Bitter-type magnet (S. Bitter, Review of Scientific Instruments, 10, 373, 1939) was employed as the magnetic field means to generate the desired intense magnetic shaping field. Such Bitter magnet was connected in series with a capacitor bank charged to 13.5 kilovolts. At such time as the magnet was operated the peak current flowing through the magnet coil equaled 55,000 amperes with a rise time of 300 microseconds. The average flux density within the coil was approximately 200,000 gauss. A 2.25-inch diameter Lucite mandrel into which a helical corrugation of four turns to the inch had been machined to a depth of 0.125 inch provided the mandrel form over which a -inch length of copper tube of 2.19-inch outside diameter and 0.06-inch wall thick ness was disposed for forming. After operation of the magnet and the forming of the copper tube material about the mandrel such combination material and mandrel assembly was allowed to cool before being removed from within the magnet coil. The Lucite mandrel was dissolved from within the formed material with acetone. Upon inspection the copper tube was found to have conformed perfectly to the corrugations on the mandrel surface. It was estimated that the magnetic field exerted a uniform force of approximately 20,000 p.s.i. upon the surface of the copper tube material.

While the invention has been disclosed herein with respect to a single preferred embodiment and specific example it will be apparent that numerous variations and modifications may be made within the spirit and scope of the invention. For example, although the solenoid coil for producing the magnetic field has been previously described herein as being mounted concentrically about the material being worked whereby the force produced acts in a radially inward direction, various alternatives are possible. Such solenoid could instead be mounted alongside or within the material to be formed, with a resulting perpendicular or radially outward force being produced to effect the forming of the material to a suitable die e.g., a flat die or a female instead of a male die or mandrel as hereinbefore described. Thus it is not intended to limit the invention except by the terms of the following claims.

What is claimed is:

1. In a process for shaping electrically conductive materials the steps comprising placing said material in circumjacent spatial relation about a mandrel of a shape desired to be formed, and applying a high intensity pulsed time varying magnetic field against the surface of said material to create radial pressure thereagainst to force said material against said mandrel and thereby conform the material to the mandrel shape.

2. In a process for shaping electrically conductive materials the steps comprising disposing said material in close spatial relation to a mandrel of desired shape, placing the combination assembly of said material and mandrel in close spatial relation to a radially restrained solenoid, and electrically energizing said solenoid to create a high intensity time varying magnetic field in the region between said solenoid and said material to create radial pressure therebetween and mold said material to said mandrel.

3. In a process for shaping electrically conductive materials the steps comprising disposing said material coaxially about a mandrel of a desired shape, placing the combination assembly of said material and mandrel concentrically Within a radially restrained solenoid, and electrically energizing said solenoid to produce a singlepulsed time-varying magnetic field between said material and solenoid to create radial pressure therebetween and urge the material inward against the mandrel to conform the material to the mandrel shape.

4. In a process for shaping electrically conductive materials the steps comprising forming a mandrel of the desired shape, disposing the material to be shaped in circumjacent spatial relation to said mandrel, placing a solenoid in circumjacent spatial relation to the surface of the material opposite to the surface facing the mandrel, placing a structural member in circumjacent spatial relation to said solenoid to restrain same from radial motion, electrically energizing said solenoid to produce a series of time-varying magnetic field pulses between said solenoid and said material to create a series of pressure pulses against said material to heat and mold same into conformation with said mandrel, and removing the mandrel from the material.

References Cited in the file of this patent UNITED STATES PATENTS 1,654,936 Jones Jan. 3, 1928 2,022,234 Everett Nov. 26, 1935 2,047,555 Gardner July 14, 1936 2,976,907 Harvey Mar. 28, 1961 

1. IN A PROCESS FOR SHAPING ELECTRICALLY CONDUCTIVE MATERIALS THE STEPS COMPRISING PLACING SAID MATERIAL IN CIRCUMJACENT SPATIAL RELATION ABOUT A MANDREL OF A SHAPE DESIRED TO BE FORMED, AND APPLYING A HIGH INTENSITY PULSED TIME VARYING MAGNETIC FIELD AGAINST THE SURFACE OF SAID MATERIAL TO CREATE RADIAL PRESSURE THEREAGAINST TO FORCE SAID MATERIAL AGAINST SAID MANDREL AND THEREBY CONFORM THE MATERIAL TO THE MANDREL SHAPE. 